This invention relates to an Extended Definition Television (EDTV) system that allows transmission of a video signal compatible with existing NTSC television receivers. In particular, the invention relates to an EDTV transmission system which is compatible with the existing NTSC system and in which the signals are transmitted for reception by a standard NTSC television receiver in a letter box format.
The current operating standard for television transmissions in the United States is defined by the National Television Systems Committee (NTSC). This standard was approved for monochrome transmission by the Federal Communications Commission in 1941 and further expanded for NTSC color television systems in 1954. This NTSC standard allocates a 6 megacycle bandwidth for each television channel, i.e. transmission of sequential video images and sound associated with one channel.
The frequency allocation of a typical television channel broadcasting in the frequency band between 54 and 60 megacycles (i.e. channel 2) using the NTSC standard is illustrated in FIG. 1. As shown, the frequency of the amplitude modulated primary picture carrier is 1.25 megacycles above the lowest frequency of the channel, i.e. 54 megacycles and the color sub-carrier is 3.58 megacycles above the primary picture carrier. The frequency of the frequency modulated (FM) audio carrier is 4.5 megacycles above the primary picture carrier. Given the 6 Mhz bandwidth of the channel, this places the audio (sound) carrier 0.25 megacycle (or 250 KHz) below the high end of the channel, located at 60 Mhz.
The amplitude modulated primary picture carrier is vestigial sideband modulated by the video components. Video signals from 0 up to about 1 MHz are double sideband modulated whereas frequencies higher than about 1 MHz up to the 4.2 MHz limit are single sideband modulated. In fact, all modulation frequencies higher than 1.25 MHz are single sideband, including the audio portion of the signal.
The standard NTSC TV camera-transmitter combination sequentially transmits a stream of two "interlaced" fields forming a single image frame using the frequency spectrum described above. Each of these two fields is made up of sequentially transmitted lines. The aspect ratio of the entire image, defined as the ratio of the height of the image to the width of the image is 3 to 4.
The first field has 262 1/2 vertically equispaced lines, and is generated every 1/30th of a second. The second field, displaced in time by 1/60th of a second from the first field, and vertically by a half line, is also transmitted every 1/30th of a second. Of the 262 1/2 lines in each field, 240 lines represent the information to be displayed in the actual video image while the balance of the field is used to transmit various types of synchronizing signals. The lines in both fields are representative of the image presented to the camera at the time of transmission of the two fields. Each line contains information detailing the value of intensity of the image over the width of the line, at the corresponding position of the line in the image.
The two fields are referred to as "interlaced" because the first field fits in the voids of the second field. The sequential reproduction of groups of two such interlaced fields reproduced every 1/30th of a second on the screen of an NTSC receiver forms a television image capable of showing the information contained within the transmitted lines making up the individual fields. The NTSC color television system uses a scan rate of 15,734,264 lines per second by the TV camera, as well as the TV receiver, to transmit and display sequences of video images respectively. Therefore, the NTSC system has a vertical resolution corresponding to 240 lines per field.
Newer systems associated with High Definition Television (HDTV) set higher standards of resolution for the transmission of television images. Currently such HDTV television cameras typically generate fields every 1/60th of a second, and have an aspect ratio of 16 to 9, with each field being made up of 525 lines, twice as many as the 262.5 lines per field used by the NTSC standard receivers. This implies a two fold improvement in image vertical resolution as compared with the NTSC system. Furthermore, the horizontal resolution is also at least double that of NTSC.
While HDTV systems deliver a better TV image, the transmission of the HDTV image is incompatible with existing NTSC receivers. Therefore, a problem remains as how to transition to this better quality HDTV system in the face of the large existing installed base of incompatible NTSC receivers. It is clearly desirable to transition from the NTSC system to the HDTV system in a smooth, gradual way where an intermediate standard compatible with both the NTSC and the HDTV system allows use of the existing NTSC receivers while also providing some the benefits of the higher image quality of the HDTV transmissions.
Such a system, designed as an intermediate step to minimize the impact of the transition to HDTV is generally identified as an Extended Definition Television (EDTV) system. Many different EDTV systems capable of transmissions compatible with the NTSC standard as well as preserving some of the advantages of the HDTV image quality have been proposed. These EDTV systems can be generally classified into three major types: a) sidepanel; b) letter box; and c) an intermediate combination of a) and b)
Typically in a sidepanel type EDTV system, the 3 by 4 aspect ratio of the NTSC transmission is preserved and the usual interlaced fields of 240 active lines each are displayed on an NTSC receiver. To create the EDTV image for display on an EDTV receiver, a left and a right sidepanel are added to the standard NTSC image to convert the NTSC 3 by 4 aspect ratio image to the EDTV 9 by 16 aspect ratio image. The sidepanels, comprising video information not displayed by the NTSC receiver, are in essence "added" to the original NTSC image to stretch it to create the 9 by 16 aspect ratio when viewed on an EDTV receiver. Such systems have been discussed in the literature, for example Kawai et al., "A Wide Screen EDTV", IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, August 1989, pg. 133-141.
This type of EDTV system however has some limitations. Since the received sidepanel images entails a different signal to noise ratio as compared to the received NTSC image, the image quality of the sidepanels and the NTSC central portion will be different. When the two side panels and the NTSC portion are combined next to each other on the same screen in the EDTV receiver, the different image quality resulting from the different signal to noise ratios of the three separate parts of the three part image becomes unpleasantly apparent.
Yet another problem associated with the side panel type transmission system is that the information displayed by the sidepanels is lost when viewed on an NTSC receiver. In effect, the NTSC receiver only displays the NTSC portion of the EDTV image, the sidepanel video information being totally lost. This loss of video information can create a variety of difficulties. For example, if a tennis match is viewed, the NTSC portion of the image may show only the net in the center and not the players at the sides.
In contrast, a letter box type EDTV transmission presents an image on a standard NTSC receiver that contains all of the lower frequency information in the full height and width of the originally transmitted image and maintains the original 9 by 16 aspect ratio of the EDTV signal. To fit the 9 by 16 aspect ratio EDTV image into the 3 by 4 ratio of the display of the NTSC receiver requires the introduction of top and bottom mask areas in the EDTV transmitted image reproduced by the NTSC receiver. In the NTSC receiver, these mask areas take the form of an upper and a lower horizontal imageless band above and below the active image area. These two bands restrict the actual video information presented to a subset of the 240 active lines per field used in a typical NTSC system. Due to geometric considerations, displaying a 9 by 16 aspect ratio image on a screen with a 3 by 4 aspect ratio requires that the number of lines used by the NTSC receiver to display the EDTV image be 180. This can be derived by considering that a 9 by 16 aspect ratio image when displayed on a 3 by 4 aspect ratio screen will use exactly 75% of the height of the 3 by 4 screen. Since the full height NTSC image contains 240 active lines, 75% of that is 180 lines. These 180 lines are the most that are available to display the complete 9 by 16 EDTV image on a standard 3 by 4 NTSC receiver.
Prior art letter box systems transmit the information required for the full EDTV image in analog form in the top and bottom mask areas around the image just before and after the transmission of the NTSC compatible portion of the image. This yields a more uniform EDTV image as compared with the side panel system when viewed on a suitable EDTV receiver because the EDTV image is reproduced from video information transmitted through the same part of the transmission channel as the NTSC signal insuring a large degree of signal uniformity. Since a large percentage of overall video information contained in the EDTV image is blended to create the image when reproduced in an EDTV receiver, marked distinctions in signal to noise ratio of the received video signal for various parts of the image are not readily apparent. A typical letter box type system has been described by Norihiro Suzuki et al, in "Experimental Hardware for Proposed Letter Box Wide Aspect EDTV", SMPTE Technical Conference, October 1990, New York City and W. F. Schreiber et al., "Single Channel HDTV System, Compatible and Non-compatible", presented at 2nd HDTV workshop, Italy, March 1988, as well as by Y. Kanatsugu et al., "Development of MUSE Family Systems", IEEE Transactions on Consumer Electronics, Vol. 35, No. 3, August 1989, pp. 153-158.
An EDTV system has also been proposed in which additional information is transmitted in analog form on a quadrature channel to modulate the primary picture carrier in the frequency spectrum limited by the 1.25 Mhz lower channel limit. See, "An Extended Definition Television System using Quadrature Modulation of the Video Carrier with Inverse Nyquist Filter", by Yashio Yasumoto et al, published in the IEEE Transaction on Consumer Electronics, CE-33, Number 3, August 1987, pgs. 146-153. This quadrature modulation cannot be decoded by a standard NTSC receiver and is therefore "invisible" to the NTSC receiver.
These analog EDTV systems transmit and process information in the same general "analog" form as the original NTSC signal. The amount of information that can therefore be transmitted is limited, thereby limiting the quality of the EDTV image when viewed with an EDTV receiver.
Others have proposed various hybrids, or intermediate combinations of the sidepanel and the letter box systems for EDTV systems whose characteristics are meant to achieve various goals through the combination of features from both the letter box and sidepanel types. Such an intermediate system is described by Minoru Ashibe, et al "A Wide aspect NTSC Compatible EDTV System" SMPTE Technical Conference, October 1990, New York City.
In view of the present limitations of EDTV television systems, it is an object of the present invention to provide an EDTV letter box system whose video image has a more uniform appearance as compared to a sidepanel system and has better quality image as compared to existing EDTV letter box systems.
It is yet another object of the present invention to provide an apparatus and method for digitally encoding and transmitting an EDTV signal that can be displayed by an NTSC receiver as well as provide additional enhancing video information for display on the EDTV receiver.
It is a more specific object of the present invention to provide an EDTV letter box system in which the enhanced additional information required in the television image is encoded and transmitted in digital form so as to create improved resolution of the EDTV image.